The invention comprises improvements in a molten solder dispenser of the kind disclosed in copending U.S. patent application Ser. No. 08/786,562, filed Jan. 21, 1997, entitled "Valveless Diaphragm Pump For Dispensing Molten Metal". That application is assigned to the assignee of the present invention.
The solder-dispensing pump disclosed in the patent application identified above includes an adjustable throttle that provides a variable flow area for a molten metal flow orifice between a pumping chamber and a reservoir for the molten metal. The pump comprises a diaphragm that is actuated pneumatically. Displacement of the diaphragm changes the effective volume in the pumping chamber, thereby creating flow of molten metal in one direction through metal dispensing nozzles and in the other direction toward the reservoir. The maximum diaphragm displacement is controlled by a stop. Molten solder delivered by the nozzles in this fashion, as molten metal is deposited on a substrate does not require pre-heating nor post-heating of the substrate in the manufacture of integrated circuit boards and the like. It is also possible, using a molten solder pump of the type disclosed in the co-pending application to avoid problems due to the temperature of the molten solder, which may exceed 200.degree. Centigrade. Deterioration of rubber seals and thermal expansion mismatches in the elements of the pump are some of the problems that are avoided.
The control of the molten solder through the solder-dispensing nozzles of a pump of the type disclosed in the co-pending application requires precision control of an adjustable throttle orifice. Such precision control is difficult to achieve because the flow of molten solder through the nozzle orifice is very sensitive to adjustments of the throttle. It is difficult with a single throttle adjustment to control the instant the liquid solder flow is cut off. Unless the flow is precisely controlled at the cutoff instant, dripping of molten solder or dribbling will occur following the cutoff as a result of the downward momentum of the dispensing stream of molten solder. A separate solenoid valve cannot be used to achieve throttle control because the variability in throttle actuation time is larger than the desired time lag between the actuation of the diaphragm and the actuation of the throttle.
The flow of molten solder through the nozzles is interrupted when the diaphragm hits the stop in the pump disclosed in the co-pending application as the upward momentum of the solder in the throttle opposes the downward momentum of the solder in the nozzles. If the upward momentum of the solder in the throttle is less than the downward momentum of the solder in the nozzles, the solder will continue to dribble from the nozzles for a brief period before the interface between the solder and the surrounding gas at the nozzle exits recedes.
It further is difficult with a pump such as that disclosed in the co-pending application to control the upward momentum of the solder so that the solder/gas interface at the nozzle exits will not rise above the entrance of the nozzle. This would generally occur when the throttle opening is too large. The upward momentum of the solder in the throttle then would be much larger than the downward momentum of the solder in the nozzles at the moment that the diaphragm hits the stop. The retraction of the solder/gas interface is further accelerated by surface tension forces. Ambient gases, if they are drawn into the pumping chamber through the nozzle following interruption of the dispensing of the molten solder, could result in pump malfunction since trapped gas would tend to compress and decompress slowly during the dispensing cycle, thereby causing further dribbling or dripping of molten solder at the nozzle exits. Such dribbling or dripping of molten solder from the nozzles following interruption of the liquid nozzle flow would tend to deposit small satellite drops on the substrate surface which could lead to shorts across the leads of the integrated circuit device.